In ReqApproval by BPM Worklist, updated the Workflow and the existing PR not generating approval

Summary:

In Requisition Approval's in BPM we updated the existing Workflow entirely and also updated the earlier Categories to new Parent categories in "Manage Procurement category hierarchy", but the existing Purchase requisition is NOT generating the approval hierarchy in "View Approvers". What can be the root cause and how to fix it?

Error in Requisition Approval > "View Approvers" → "We can't assign approvers to this requisition right now."